This invention relates to a thermistor element having resistance with negative temperature coefficient (hereinafter referred to as an NTC thermistor element) and more particularly to an improvement in the kinds of NTC thermistor element having a plurality of inner electrodes inside its thermistor body.
NTC thermistor elements are widely in use for detecting the temperature of the atmosphere, solid and liquid materials, as well as for compensating for changes in the characteristics of a circuit or its component due to temperature variations. As disclosed in Japanese Patent Publications 4-130702 and 62-137804, for example, prior art NTC thermistor element chips may be of a face-to-face type having electrodes disposed opposite each other in a coplanar relationship or a layered type having a plurality of inner electrodes disposed one above another inside the thermistor body in a layered formation.
FIG. 11 shows a prior art NTC thermistor element 61 of a face-to-face type having a thermistor body 62 obtained by sintering a plurality of transition metal oxides such as nickel oxide and cobalt oxide, containing therein inner electrodes 63 and 64 opposite each other at a certain height with a specified gap therebetween. An outer electrode 65 is formed over one end surface (on the left-hand side) of the thermistor body 62 and connected to one of the inner electrodes 63, and another outer electrode 66 is formed over the other end surface (on the right-hand side) of the thermistor body 26 and connected to the other inner electrode 64. The resistance value of this NTC thermistor element 61 is determined by the gap between the mutually opposite inner electrodes 63 and 64. Since the two inner electrodes 63 and 64 are in a coplanar relationship, the resistance value of the NTC thermistor element 61 can be controlled to a high degree of accuracy by accurately forming these inner electrodes 63 and 64 on a so-called green sheet which is used for obtaining the thermistor body 62.
FIG. 12 shows another example of prior art NTC thermistor element 67 of the face-to-face type characterized as having other pairs of inner electrodes 68a, 68b, 69a, 69b, 70a and 70b in addition to the electrodes 63 and 64 as shown in FIG. 11, that is, four pairs of mutually opposite electrodes at four different heights inside the thermistor body.
FIG. 13 shows an NTC thermistor element 71 of a layered type having a plurality of inner electrodes 73, 74 and 75 disposed overlappingly one above another through thermistor layers inside a thermistor body 72. Inner electrodes 73 and 75 are connected to an outer electrode 76 formed over one end surface of the thermistor body 72, and inner electrode 74 is connected to another outer electrode 77 formed over the other end surface of the thermistor body 72. With this NTC thermistor element 71, the resistance value is determined by the separations between the upper and lower inner electrodes 73 and 75 and the middle inner electrode 74. Thus, a thermistor element with a small resistance value can be more easily obtained by this type.
In summary, prior art NTC thermistor elements of the face-to-face type, as shown at 61 and 67, are advantageous wherein their resistance values can be accurately controlled but it is difficult to reduce their resistance values. The resistance value can be reduced by reducing the gap between the mutually opposite pair of inner electrodes (such as between electrodes 63 and 64) but the possibility of occurrence of a short circuit increases if the gap is reduced excessively. In other words, there is a limit beyond which the resistance value of an NTC thermistor element cannot be reduced. Another problem is that edge portions of the outer electrodes 65 and 66 extending in the direction of a line connecting the two end surfaces serve as parallel resistors with the inner electrodes, and their effect on the total resistance value is not negligible.
With an NTC thermistor element of the layered type, such as shown at 71, the resistance value can be reduced by increasing the number of layers of the inner electrodes, but there are fluctuations in the thickness of green sheets which are used for the production, and the resistance value may vary significantly, caused by such fluctuations as well as the accuracy in overlapping the green sheets. In other words, although NTC thermistor elements with low resistance values can be obtained, the more the resistance value is reduced, the greater becomes the variation in the resistance value.